All right, let's bring in NPR's counter-terrorism correspondent Dina Temple-Raston, to update us on the investigation into the Boston Marathon attack.

And, Dina, we just heard from Corey Flintoff all about the Russian Republic of Dagestan. And U.S. officials have been there already to see if there are leads to follow.

DINA TEMPLE-RASTON, BYLINE: Well, they're trying to build a timeline to account for the six months that Tamerlan Tsarnaev was in Dagestan. And they're looking for associations he might have had. There is, for example, a Russian-born Canadian militant named William Plotnikov that they are checking into. And officials said they're trying to establish whether he had some sort of relationship with Tamerlan Tsarnaev. This Canadian was a boxer. He overlapped with Tamerlane in Dagestan last summer, and he was killed by Russian police just days before Tamerlan return to the U.S. last July.

Officials are also continuing to interview two Kazakh students who were arrested here in the U.S. in New Bedford, Massachusetts soon after the bombing. They were friends with the brothers. In fact, they're in a photograph with Dzhokhar, the younger brother, taken last year in Times Square. They're being held on immigration charges. But officials are trying to figure out if they knew about or helped with the attack.

GREENE: So, Dagestan, as we've heard of the place where there are militants. There is anger. And I guess they're trying to find out what if any contacts or influences these brothers might have had there.

TEMPLE-RASTON: Exactly.

GREENE: Well, OK. So that's the situation in Dagestan. As we said, this investigation is spanning many countries. In Boston, Dina, the focus has shifted to a woman's DNA that was found on one of the bombs. What could this mean?

TEMPLE-RASTON: Well, law enforcement officials, among other things, they collected DNA samples from Katherine Russell. She's the widow of the accused bomber, Tamerlan Tsarnaev. And if her DNA matches what they found on the bomb that might indicate that she had some direct contact with the device.

It could be significant although it could have an innocent explanation, as well. There are innocent ways a hair, for example, could have ended up on the bomb. So it doesn't prove anything. It just gives them another line of inquiry. And there's a chance that the DNA isn't hers. And if it isn't hers, that could lead to another co-conspirator.

And officials told us that there are as many as a dozen people they're investigating in connection with this. And those are the kinds of things that investigators are pursuing. They also went to a landfill near the younger brother dorm room over the weekend. And they were hoping to find a laptop there. Apparently they didn't find one but they did find a cell phone and some other material they told us might be helpful to the investigation.

I mean, one of the big things that interest them is where the bombs might've been tested. Officials say it would've been really unlikely that a device was wired up without testing it, and they haven't been able to find out yet.

GREENE: It sounds important to note that there could, could be more people involved here beyond the two brothers. One brother, of course, Tamerlan, is dead. Dzhokhar Tsarnaev still alive in a prison hospital. Are investigators learning anymore from him at this point?

TEMPLE-RASTON: Well, we're told that after he was read his Miranda rights last week, he hasn't spoken with investigators in a substantive way since. He allegedly told them last week, before he exercised his right to remain silent, that he and his brother were working alone. But there were some details he had given officials that just didn't check out. So they're trying to figure out if there were people who helped them before or after.

And there's some question of whether or not there were discussions about taking the death penalty off the table in exchange for cooperation. But it's unclear whether investigators me that offer - to try to get them to talk - or if his lawyers asked if that was possible.

GREENE: OK, this is a story we'll keep following. NPR's Dina Temple-Raston updating us on the investigation in Boston. Thanks a lot, Dina.

TEMPLE-RASTON: You're welcome.

GREENE: And you're listening to her on MORNING EDITION from NPR News. Transcript provided by NPR, Copyright NPR.